The present invention generally relates to a method and an apparatus for testing the operation of a cassette pod for storing and transporting semiconductor wafers and more particularly, relates to a method and an apparatus for testing the operation of a cassette pod door or the unlatching torque of a cassette pod door during a door opening procedure.
The high level of automation used in fabricating semiconductor devices relies on sophisticated handling and transport equipment for moving semiconductor wafers between various processing stations. Most handling and transport operations are conducted under automatic control using a programmed computer which issues control signals for operating the equipment with little or no intervention by an operator. In many systems, standard mechanical interface (SMIF) pods are used to transport batches of wafers that are stored in cassettes. These pods include a base upon which the cassettes rest, and a cover removably secured to the base and completely enclosing the cassette. The cover protectively surrounds the cassette, and thus the wafers, from the surrounding environment which may contain airborne, contamination particles. SMIF pods are most often used to transport cassettes from one clean room environment to another, where during the transport movement, the wafers, if not covered, are exposed to the contaminating environment. In more recent development, FOUPs (front open unified pod) are used for storing and transporting 12xe2x80x3 wafers.
After a pod has reached the vicinity of a processing station within a protected, clean room environment, it is necessary to remove the cover so that automated wafer transfer robots can access the individual wafers held in the cassette. The cover is held on the pod by one of various types of latching mechanisms which is automatically actuated to latch and unlatch the cover by means of actuating controls positioned at each processing station. Thus, when a pod is transported to a processing station, control mechanisms engage the latch mechanism on the pod to unlatch the cover, following which either an operator or a robotic mechanism removes the cover to expose the cassette. After the batch of wafers in the cassette is processed, the cover is reinstalled on the pod base, either manually or robotically, and the control mechanism is again actuated to latch the cover onto the base (for SMIF) or onto the back (for FOUP) before the pod leaves the processing station.
In spite of the fact that positive latch mechanisms are designed to lock the cover on the pod base, occasions arise when, for a variety of reasons, the latch fails to lock the cover onto the pod base. This may occur, for example, when a foreign article becomes lodged between the cover and the base or when the cassette becomes tilted on the base, thus interfering with the proper seating of the cover. In other cases, the control mechanism for actuating the latch may malfunction. Frequently, such failure of the latch mechanism to lock the cover on the pod base goes undetected by process operators. As a result, it is possible that contaminants may pass through between the pod base and cover when the pod leaves the clean room environment, resulting in possible contamination of the wafers.
Accordingly, there is a clear need in the art for an improved pod construction which insures that the cover is properly locked onto the pod. The present invention is directed toward satisfying this need in the art.
FIG. 1 illustrates a cassette pod such as a FOUP that is installed on a process machine. The cassette pod 10 is positioned on a loadport 12 of the process machine 14. The loadport 12 is normally equipped with a plurality of locating pins 16 for the proper positioning of the cassette pod 10.
A detailed perspective view of the cassette pod, i.e. the FOUP 10, is shown in FIG. 4. The FOUP 10 is constructed by a body portion 18 and a cover portion 28. The body portion 18 is provided with a cavity 46 equipped with a multiplicity of partitions 48 for the positioning of 25 wafers of the 300 mm size. The body portion 18 is further provided with sloped handles 50 on both sides of the body for ease of transporting. On top of the body portion 18 is provided a plate member 52 for gripping by a transport arm (not shown) of an overhead hoist transport system. It should be noted that, for simplicity reasons, the latching mechanism for opening and closing the FOUP door, or the cover portion 28, is not shown in FIG. 2.
In the conventional cover member, or door 28 of the FOUP 10, the door opening/closing mechanism is shown in FIG. 3 in a plane view. Locking tabs 20a, 20b, 22a and 22b are provided at the top and at the bottom of the cover member 28. Circular drive plates 24a and 24b are connected to the locking tabs 20a-22b by mechanical linkage 26a and 26b and operated by latch holes 28a and 28b. A pair of latch keys (not shown) that are part of the door opener mechanism of the loadport (not shown) are inserted into the latch holes 28a and 28b and turned by a DC motor to lock or unlock the cover member 28 to the loadport opening. As shown in FIG. 3, when the drive plates 24a and 24b are turned in a clockwise direction, the locking tabs 20a, 20b, 22a and 22b are withdrawn into the frame of the cover member 28 and therefore, unlocking the cover member 28 from the body member 18 of the cassette pod 10 (shown in FIG. 2). Conversely, when the drive plates 24a and 24b are turned by the DC motor in a counter-clockwise direction, the locking tabs 20a, 20b, 22a and 22b are extended out of the frame of the cover member 28 to engage receptacles (not shown) in the body member 18 and thus, locking the cover member 28 to the body member 18 of the cassette pod 10.
Problems occur when operating the conventional door opening/closing mechanism shown in FIG. 3 installed on a FOUP. For instance, the latch keys (not shown) that are turned by the DC motor sometimes are misaligned with the latch holes 28a and 28b and thus, the door opening or closing operation cannot be carried out. Secondly, the torque of the DC motor cannot always be precisely controlled such that the angle of turning, i.e. such as a 90xc2x0 angle, cannot be precisely controlled. A door opening or closing mechanism can not be completely executed when the angle of rotation is not exactly 90xc2x0. For instance, during a door locking operation if the latch holes 28a and 28b are not turned to a perfect vertical position in order to lock the cover member 28 to the body member 18 and when the FOUP 10 is later positioned on another process equipment, a similar set of door opening latch keys can not be inserted into the latch holes 28a and 28b and thus, the cover member 28 cannot be opened or unlocked from the body member 18.
In the operation of cassette pods for the 300 mm diameter wafers, i.e. the FOUP, an error message is frequently observed during the unlatching of a FOUP door from the FOUP. The error message occurs when a torque required to open the FOUP door is detected to be larger than 80 in-lb. A higher than normal torque reading indicates one of many possible defective conditions of the FOUP. For instance, a distortion in the latch mechanism of the FOUP door and thus, excessively high torque is required to unlatch the door from the FOUP. Other defective conditions may include a misalignment between the latch key of the door opener in the loadport and the latch holes in the FOUP door. The misalignment may also be caused by a distorted FOUP door which occurs after extended usage.
Conventionally, a DC motor is used to unlatch the FOUP door. When a torque reading of the DC motor, during the unlatching operation, exceeds 80 in-lb, the DC motor is stopped to prevent any possible damage that may be caused in the motor. The FOUP is then manually removed from the loadport for the operator to diagnose the cause of the malfunction. The manual operation of removal can be difficult since the FOUP door may be partially unlatched and thus, accidental opening of the door may cause severe damage to the wafers stored inside.
It is therefore an object of the present invention to provide a method for unlatching a cassette pod door from a pod without the drawbacks or shortcomings of the conventional method.
It is another object of the present invention to provide a method for unlatching the cassette pod door of a pod that can be carried out without causing damage to the door or the wafers stored inside.
It is a further object of the present invention to provide a method for testing the operation of a cassette pod door that can be used as a reliability screening test for detecting defective cassette pod doors.
It is another further object of the present invention to provide a method for testing the unlatching torque of a cassette pod door for detecting doors that require excessive torque for opening.
It is still another object of the present invention to provide a method for testing the unlatching torque of a cassette pod door by first detecting an excessively high torque and then reversing a motor to re-latch the door into a secure, locked position before the pod is removed.
It is yet another object of the present invention to provide an apparatus for testing the unlatching torque of a cassette pod door that can be used as a reliability screening test.
In accordance with the present invention, a method and an apparatus for testing the unlatching torque of a cassette pod door from a cassette pod and thus, screening out defective cassette pod doors are provided.
In a preferred embodiment, a method for testing the operation of a cassette pod door can be carried out by the operating steps of providing a cassette pod that has a door latched on an opening; engaging a latch key to a latch hole on an exposed surface of the door; switching on a DC motor to turn the latch key in a first rotational direction for unlatching the door from the cassette pod; measuring a torque of the DC motor during the switching-on step; stopping the unlatching operation by switching-off the DC motor when the torque measured exceeds 80 in-lb; and switching on the DC motor to turn the latch key in a second rotational direction opposite to the first rotational direction to re-latch the door onto the cassette pod.
The method for testing the operation of a cassette pod door may further include the step of, during the second switching-on step, increasing the torque to higher than 80 in-lb to turn the latch key in the second rotational direction. The method may further include the step of providing a cassette pod for holding 300 mm diameter wafers. The method may further include the step of increasing the torque by increasing a current input to the DC motor, or the step of providing a controller for controlling the operation of the DC motor. The method may further include the step of sending an alarm signal to an operator from the controller when a torque larger than 80 in-lb on the DC motor is detected. The method may further include the step of marking the cassette pod as defective when the torque measured exceeds 80 in-lb. The method may further include the step of detecting a position of the latch hole and determining whether the cassette pod door is completely unlatched, or the step of detecting a position of the latch hole and sending a signal to a controller to indicate that the cassette pod door is unlatched.
The present invention is further directed to an apparatus for testing the unlatching torque of a cassette pod door which includes a stage onto which a cassette pod having a door latched thereon is positioned; a door opening mechanism including latch keys on the stage; a DC motor for turning the latch keys after engaging latch holes situated in the cassette pod door in either a latching direction or an unlatching direction; and a controller for measuring a torque on the DC motor by monitoring a current input to the DC motor, the controller stops an unlatching motion of the latch keys when a torque measured is greater than 80 in-lb and causes the latch keys to turn in an opposite direction to re-latch the door on the cassette pod before the pod is removed from the apparatus.
The apparatus for testing the unlatching torque of a cassette pod door may further include an alarm for triggering by the controller when the torque of larger than 80 in-lb is detected. The apparatus may further include a position detector mounted juxtaposed to the latch hole for determining an unlatched position of the latch keys, or a position detector mounted juxtaposed to the latch hole for sending out a signal indicative of an unlatched position to the controller when the position is detected. The apparatus may further include a position detector mounted juxtaposed to the latch keys for determining an unlatched position of the latch keys, or a position detector mounted juxtaposed to the latch keys for sending out a signal indicative of an unlatched position to the controller. The controller converts an input current to the DC motor to a measurement of torque. The apparatus may further include a torque display device for displaying a torque measured by the controller.